CS - Try reading this about how water forms temperature layers in ponds and lakes. Here is some preliminary background reading to help you better understand the advanced reading in the link which explains why ponds that form temperature layers and need aeration which in reality is just strong mixing action of bringing "bad" water from bottom to top where it is "cleaned" up (reoxygenated) and often degased of the gasses from decomposition (organic decay). Each water layer has a name: top warm layer is epilimnion, middle layer is thermocline, and deep cold layer is hypolimnion. The top layer is the most well mixed layer by primarily the wind action and usually contains the most dissolved oxygen aka DO. Thermocline is the transition layer with rapid temperature change and this layer separates the top layer from the bottom layer aka hypolimnion. Hypo is from Greek meaning under and limnion meaning lake. Bottom or hypolimnion layer often has low or no DO several weeks after it is formed. It depends. Generally the older and more nutrient rich the pond the faster the thermocline and hypolimnion layers loose the DO. In a new pond DO loss is not so fast as in an older pond or a productive pond (green with blooms). Older productive ponds have more growth, thus more death, thus more dead stuff to daily decay. Usually clear water with visibilities of 5-10ft means low productivity (less growth) and they have naturally higher DO concentrations deeper into the pond due to sunlight penetrating into these deep waters where phytoplankton can thrive and make DO.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92639#Post92639For general use ponds IMO, you will not do irreparable damage to a new pond if you do not aerate it in the first year or two. There are exceptions and it all depends. The sooner you install bottom aeration the less the pond will have to "catch up" on digesting the organic accumulations that deposit on the pond's bottom sediments in the thermocline and hypolimnion areas. New ponds do not generally form lots of organic muck in the pond's "belly' during the first year or two unless the pond gets lots of nutrient or fertilizer inputs which cause abundant plant growth.
We can answer questions that you have.
IMO in the long term you will be better pleased if you make more of the bottom 10-12ft instead of 5 ft. All your weed problems will occur in the 0-6ft depths esp if the pond has fairly clear water (visibilities 3-5ft). One centrally placed diffuser can adequately mix your 0.4ac pond if the pond is round, square, or an oblate circle. the more the pond deviates from the circle shape the more it needs a second diffuser (bubbler) to adequately mix all areas of outcroppings, bays, bulges, or fingers.
